The prior art has long recognized that several parameters of engine operation influence the initiation of combustion in an HCCI engine. See, for example, U.S. Pat. No. 6,286,482 to Flynn et al., and Aceves, HCCI Combustion: Analysis and Experiments, SAE 2001-01-2077. Such recognized parameters, collectively designated herein as “engine state parameters,” include: compression ratio, intake charge temperature, oxygen concentration in the charge air, equivalence ratio, charge air density, and boost pressure. Long lacking, however, was a practical method for adjustment of these and other parameters in a way to control the timing or efficiency of HCCI combustion.
Recently, commonly assigned U.S. Pat. No. 7,237,532 to Gray et al. provided a method for maintaining stable, efficient HCCI combustion across the operating range of an engine using gasoline-like fuels, through closed-loop feedback control of a cylinder-specific combustion parameter, preferably, the maximum rate of pressure rise (MRPR) inside the cylinder. Gray et al. were the first to teach the use of MRPR for HCCI control, but their method did not overcome the prior art's dependence on the use of in-cylinder pressure transducers for MRPR measurement. Reliance on in-cylinder transducers is a substantial drawback due to (1) their high cost, poor reliability, and short life span, and (2) the heavy computational load associated with the necessary, near-continuous (high-resolution) sampling. The present invention overcomes this limitation in the prior art by providing a method for MRPR estimation based on signals from existing extra-cylinder sensors—production-engine sensors physically located outside the combustion cylinder—such as a crankshaft position sensor or knock sensor, thereby eliminating the need for in-cylinder pressure transducers, and substantially reducing computational load.
The prior art includes methods for estimating some combustion parameters without in-cylinder pressure measurements. In particular, U.S. Pat. No. 6,866,024 to Rizzoni et al. provides a method for estimating the net output torque of a combustion event, based on crankshaft dynamics and a reconstructed cylinder indicated pressure. Similarly, U.S. Patent Application 2003/0236611 by James et al. describes using measurements of crankshaft acceleration to calculate indicated mean effective pressure (IMEP), output torque, or work per cylinder. These methods, however, are limited to statistical approaches unaided by self-tuning or other intelligent guidance for robustness. Moreover, these methods do not estimate MRPR, nor do they recognize the physical significance of MRPR in characterizing HCCI combustion, thus teaching away from the use of MRPR for HCCI engine control.
The inventors have noted that, once initiated, the HCCI combustion process has a very short duration relative to combustion in a conventional spark-ignition engine; HCCI combustion thus occurs at near-constant cylinder volume. FIG. 2A shows the pressure-volume (PV) diagram for a HCCI engine cycle; the combustion (pressure rise) segment is shown expanded in FIG. 2B. Cylinder pressure rise during HCCI combustion traces approximately a straight line, as a function of cylinder volume, with a sharp, near-vertical slope (FIG. 2B); the same is true when cylinder pressure is plotted as a function of crank angle position (FIG. 2C, PΦ diagram). This feature of the curves in FIG. 2 captures several critical characteristics of HCCI combustion, as noted by the inventors. First, all HCCI events can be described as “quasi-constant volume” combustion. Second, because the maximum rate of pressure rise (MRPR) is a straight-line slope—rather than the point of maximal curvature as is the case for conventional spark-ignition (SI) combustion—it can be used to characterize HCCI burning. Third, the MRPR value is a robust indicator of HCCI combustion status. In particular, MRPR values below a lower limit (around 2 bar/deg) indicate misfire, while values above an upper limit (around 12 bar/deg depending on engine type) indicate knock; between these limits, HCCI combustion is stable, MPRR is proportional to IMEP for any given engine operating range (see FIG. 2D), and MRPR varies in proportion to fuel quantity.
MRPR thus proves to be a uniquely rich source of information about HCCI combustion physics, and is important for HCCI engine transient control: no other single combustion parameter carries the same wealth of information, or captures HCCI combustion status with the same robustness. By way of example, FIG. 2D shows the relationship between MRPR and IMEP (a combustion parameter of considerable focus in the prior art), for different states and speed/load points of a HCCI engine. For a given speed/load point, IMEP and MRPR are highly correlated because of the quasi-constant volume behavior of HCCI combustion. However, while MRPR limits on stable HCCI combustion are fixed across the engine's operating range, the stable-combustion range of IMEP values varies widely depending on engine state. Prior art methods for crankshaft-based estimation of combustion characteristics, however, have focused exclusively on IMEP and similar parameters, and away from MRPR.